The project has been extended to study the molecular nature of the mouse genome containing the genetic locus Fis-1. This locus has been shown to be the site of integration of Friend MuLV in some lymphomas appearing after inoculation of several inbred mouse strains with the ecotropic Friend MuLV. In about 5% of approximately 100 tumors examined, copies of Friend MuLV were found integrated within a 1.5 kb region of the mouse genome (Fis-1). From a mouse genomic library constructed in lambda virus, 35 kb of mouse genomic sequence surrounding the Fis-1 locus were isolated. These mouse sequences were present in two recombinant lambda phages. The mouse sequence inserts in the lambda phage were subcloned into plasmids and fragments from 7 to .3 kb were obtained. Each subcloned fragment was analyzed for unique or repetitive mouse sequence content by Southern blot analysis. Subclones containing a total of about 15 kb of unique mouse sequence were further utilized to test for expression in RNA isolated from mouse tumors. No RNA expression has been detected in normal mouse tissue or in mouse tumors (either with or without Friend MuLV insertions in the Fis-1 region). Sequence determination of the 1.5 kb Fis-1 sequence is in progress.
